Process for treating a liquid-wet polycarbonate membrane to improve its gas separation properties

Abstract

This invention relates to a method of treating liquid-wet polycarbonate membranes to improve gas separation properties.

What is claimed is: 
     
       1. A process for treating a liquid-wet polycarbonate membrane comprising the steps of: A. contacting at least one side of a polycarbonate membrane with water at a temperature which does not deleteriously affect the integrity of the membrane under conditions such the water removes substantially all of the residual solvent and non-solvent from the membrane;   B. contacting at least one side of the membrane with air, an inert gas, or vacuum at a temperature under conditions such that substantially all of the water is removed from the membrane without deleteriously affecting the integrity of the membrane; and   C. simultaneously or consecutively heat treating the membrane under conditions such that the membrane subsequently exhibits a decrease in gas flux of less than about 2 percent over 30 days under an operating temperature of less than about 25° C. and an operating pressure of less than about 200 psig with an increase in gas selectivity of at least about 5 percent compared with a membrane which is not heat treated.   
     
     
       2. The process of claim 1 which further comprises the additional step of first storing the membrane in water under ambient conditions of temperature and pressure for a period of time sufficient to provide uniform membrane composition. 
     
     
       3. The process of claim 2 wherein the polycarbonate membrane is fabricated from polymers comprised of backbone units corresponding to the formula: ##STR11## wherein R is independently in each occurrence --CO--, --S--, --SO 2  --, --O--, a C 1-6  divalent hydrocarbon radical, an inertly substituted C 1-6  divalent hydrocarbon radical, or a C 1-6  divalent halocarbon radical; X is independently in each occurrence hydrogen, chlorine, bromine, fluorine, a C 1-4  alkyl, or a C 1-4  haloalkyl; and   n is a positive real number of about 20 or greater.   
     
     
       4. The process of claim 3 wherein R is a C 1-6  divalent hydrocarbon radical; and   X is hydrogen, chlorine, bromine, fluorine, or methyl.   
     
     
       5. The process of claim 4 wherein the polycarbonate membrane is an asymmetric or POWADIR membrane. 
     
     
       6. The process of claim 5 wherein the polycarbonate membrane is in hollow fiber form. 
     
     
       7. The process of claim 6 wherein the polycarbonate membrane is fabricated from 2,2-bis(3,5-dibromo-4-hydroxyphenyl)propane. 
     
     
       8. The process of claim 7 wherein the residual solvent and non-solvent in the membrane is less than about 0.5 weight percent. 
     
     
       9. The process of claim 8 wherein the water is at a temperature of from about 0° to 100° C. 
     
     
       10. The process of claim 9 wherein the heat treatment temperature is from about 20° to 150° C. 
     
     
       11. The process of claim 10 wherein the polycarbonate membrane is first stored in water under ambient conditions of temperature and pressure for at least 1 day. 
     
     
       12. The process of claim 11 wherein the polycarbonate membrane possesses a separation factor for oxygen/nitrogen of at least about 4. 
     
     
       13. The process of claim 12 wherein the polycarbonate membrane possesses an oxygen gas flux of at least about ##EQU10## 
     
     
       14. The process of claim 13 wherein the polycarbonate membrane possesses an oxygen flux of at least about 1.0 Barrers. 
     
     
       15. The process of claim 11 wherein the polycarbonate membrane is stored in an amount of bleach sufficient to prevent bacterial growth on the membrane without adversely affecting membrane separation properties.